Direct drive synchronous generators used in wind turbines have the inherent issue of dealing with short circuit faults, mainly due to converter short circuits, which are a high probability of happening over a lifetime. The result of the short circuit is that a large current is induced in the generator windings and causes thermal loads, magnet demagnetisation and large airgap torques. These short circuit currents can be three times higher than the operating current, resulting in airgap shear forces that can potentially damage the turbine structure.
This is circumvented by designing the structure of the generator and the wind turbine, respectively, more robust to cope with the increased fault condition loads, causing the turbine to be over designed for normal operation and adding costs to the final cost of energy (COE).
This is also done with magnets of the generator, in that the magnet height and grade are chosen to survive the short circuit field that is seen during the fault condition, again adding cost to the turbine generator.